Population Ecology

Population as a Unit of Study

• Population– A group of organisms of the same species occupying

a particular space at a particular time

• Demes– Groups of interbreeding organisms, the smallest

collective unit of a plant or animal population

Population as a Unit of Study

• Population has group and not individual characteristics

• Basic characteristics of a population:– Density (no/area; no/vol)– Size (numbers)– Age Structure (based on age distribution)– Dispersion (the spread of individuals in

relation to one another)

immigration

emigration

natalitymortality

Four basic populationparameters

DENSITY

Density: no of organisms per unit area or per unit volume

Natality: the reproductive output of a population (birth, reproduction)

Mortality: the death of organisms in a population

Immigration: the no of organisms moving into area occupied by the population

Emigration: the no of organisms moving out of the area occupied by the population

Immigration and Emigration are referred to as Migration

Population parameters

• Population parameters affect population density

Populationdensity

Natality

+

Emigration

Mortality

Immigration

-

-+

How to estimate population density?

• Techniques differ between organisms such that the technique to estimate deer cannot be applied to bacteria or protozoa or vice versa

• There are 2 fundamental attributes that affect and ecologists choice of technique for population estimation

2 attributes

Size-small animals/plants are usually more abundantthan large animals/plants

Mobility-based on movements of these organisms

Why the need to estimate population density?

• Estimates of population are made for two reasons:– How to quantify nature – ecologist role– Estimates are allows for comparisons between

different populations in terms of space and time measure

2 broad approaches to estimate pop density

Absolute densityNo of individual per area/ per volume

Important for conservation and management

Relative densityComparative no of organisms

Two areas of equal sizes, which area has more organisme.g, between area x and y

Area x has more organism than area y

Absolute density

• Making total counts and by using sampling methods

• Total counts - direct counting of populations- human pop census, - trees in a given area, - breeding colonies can be photographed then

later counted- in general total counts are possible for few

animals

Measurements of Absolute density

• Sampling methods– to count only a small proportion of the

population - sample

• Using the sample to estimate the total population

• 2 general sampling techniques:1) Use of quadrats2) Capture-recapture method

Use of quadrats

Count all individuals on several quadrats of known size, then extrapolate the average count to the whole area

Quadrat- a sampling area of any shape (may be a rectangle, triangle or circle)

3 requirements:• the pop in the quadrat must be determined exactly• area of the quadrat must be known• quadrat/s must be representative of the area • achieved by random sampling

Quadrat sampling in plant population

• Conduct a transect in the upland hardwood forest

• 3 transect line, 110 meters long, count all trees taller than 25cm within 1meter of each line

• By utilising the quadrat method sampling for old trees and seedlings, we can determine if populations were likely to change over time

Capture recapture method

Capture, marking, release, and recapture-important for mobile animalsWhy?-it allows not only an estimate of density but also estimates of birth rate

and death rate for the population being studied

Capture animal, mark (tag) them and then release them

Peterson method:Involves 2 sampling periods

Capture, mark and release at time 1Capture and check for marked animals at time 2

Time intervals between the 2 samples must be short because this method assumes a closed population with no recruitment of new individuals into the

Population between time 1 and 2 and no losses of marked individuals

Formula for capture-recapture method

Marked animals in 2nd sample = Marked animals in 1st sampleTotal caught in 2nd sample Total population size

e.g of capture recapture method

• Dahl marked trout in small Norwegian lakes to estimate the size of the population that was subject to fishing. He marked and released 109 trout, and in 2nd sample a few days later caught 177 trout, of which 57 were marked. From the data, what is the estimate population size?

e.g of capture recapture method

• By using the formula

57 = 109177 Total pop size

Total pop size = (109 X 177)57

= 338 trout

Relative density

• Traps – no caught per day per trap – animals caught will depend on their density, activity and range of movement, skill in placing traps – rough idea of abundance – night flying insects, pitfall traps for beetles, suction traps for aerial insects

• Fecal pellets – rabbits, deer, field mice – provides an index of pop size

• Vocalization frequency – bird calls per 10 mins, can be used for frogs, cicadas, crickets

• Pelt records – trapper records dates back 300 years – of lynx

Relative density

• Catch per unit effort – index of fish abundance – no of fish per cast net or no of fish per 1 hour trawling

• Number of artifacts – thing left behind – pupal cases of emerging insects

• Questionnaires – to sportsmen (eg fish)and trappers

• Cover - % ground surface covered – in botany, invertebrate studies of the rocky intertidal zone

• Feeding capacity – bait taken – for rats and mice – index of density

• Roadside counts – birds observed while driving standard distances

Natality

• The production of new individuals by birth, hatching, germination or fission

• 2 aspects of reproduction must be distinguished:

• Fecundity • fertility

Natality

• Fecundity-physiological notion that refers to an organism’s potential reproductive capacity

• Fertility-ecological concept based on the no of viable offspring produced during a period time

• Realized fertility and potential fecundity-we must be able to distinguish between them

Natality

• E.g, realized fertility rate for a human pop may be only 1 birth per 15 years per female in the child-bearing ages

• While the potential fecundity rate for humans is 1 birth per 10 to 11 months per female in the childbearing ages

Mortality

• Biologists-interested not only in why organisms die but also why they die at a given age

• Longevity-the age of death of individuals in a population

• 2 types:– Potential longevity– Realized longevity

Mortality

• Potential longevity– The maximum life span of an individual of a particular

sp is a limit set by the physiology of the organism, such that it simply dies of old age

– The average longevity of individuals living under optimum conditions

– However, organisms rarely live under optimum conditions-most die from disease, or eaten by predators or succumb to a number of natural hazards

Mortality

• Realized longevity– The actual life span of an organism– Can be measured in the field, while potential

longevity only in labs or zoos

examples

• European robin has an average life expectation of 1 year in the wild, whereas it can live at least 11 year in captivity

Population dispersion patterns

3 types

random

uniformclumped

Population dispersion patterns

• Random-when the position of each individuals in a pop is independent of the others

• Uniform-it results as a form of some negative interactions

• Common among animal pop where individuals defend an area for their own exclusive use (teritoriality) or in plant pop where severe competition exist for belowground resources, i.e water or nutrients

Population dispersion patterns

• Clumped-where individuals occur in groups

• Reason-suitable habitat or resources may be distributed as patches on a larger landscape

DEMOGRAPHIC TECHNIQUES

A technique to summarize how mortality occurs in a population

Is mortality high among juveniles?

Do older organisms have a higher mortality rate than younger organisms?

Life tables

• Developed to describe the mortality schedule of a population

• An age-specific summary of the mortality rates operating on a cohort of individuals

• Cohort-may include the entire population, or only males or only individuals born in a given year

Cohort life tables •generation or horizontal life tables •following the cohort throughout life – eg., annual seeds or lambs born.

Static life tables •stationary, time specific, current, vertical life tables•records of age at death – individuals in sample are born at different times on basis of cross section of a pop at a specific time

Age distribution •consists of proportion of individuals of different ages within a pop •can estimate survival by calculating the difference in proportion of individuals in succeeding age classes•produces a static life table and assumes that the difference in numbers between age classes is a result of mortality

• Mortality is a key parameter that drive pop change, therefore we need to quantify it in a population– It is high among juveniles, adults?

• We can quantify mortality by constructing a life table – mortality schedule of a population – age specific summary of mortality rates operating on a cohort (= a group born at the same time) (you are not required to know how to construct or calculate a life table – understand it!)

e.g of cohort life table for the song sparrow

Age in years (x)

Observed no of birds alive (nx)

Proportion surviving at start of age interval x (lx)

No dying within age interval x to x+1 (dx)

Rate of mortality (qx)

0 115 1.0 90 0.78

1 25 0.217 6 0.24

2 19 0.165 7 0.37

3 12 0.104 10 0.83

4 2 0.017 1 0.50

5 1 0.009 1 1.0

6 0 0.0 - -

Types of survivorship curves

Type 1- low type of mortality for most of the life spanand then high losses of older organisms

Humans and large mammals

Type 2- constant per capita rate of mortality independent of ageeg birds, squirrels

Type 3-High per capita mortality early in life, followed by a period of much lower and relatively constant lossFishes, invertebrates, parasites

Fig. 10.14

Fig. 10.15

Fig. 10.16

Age Distribution

Fig. 11.24

POPULATION GROWTH

Population growth

• Refers to how the number of individuals in a population increases or decreases with time (N, t)

• Reflects the difference between rates of birth and death

• in pop, if new births occur

• in pop, if death occurs

Population growthChange in pop size births during – deaths duringduring time interval = time interval time interval

If N represents pop size and t represents timethen ΔN is the change in pop size and Δt is the time interval

So, the equation:ΔN = B-D

ΔtB-the number of births in popD-the number of deaths in pop

Population growth

• Let r = b - d

• Then, the equation,

dN/dt = rN

The rate of change of population (dN/dt) is a function of r (rate of increase) and the population size (N)

Geometric Growth

• When generations do not overlap, growth can be modeled geometrically.

Nt = Noλt

– Nt = Number of individuals at time t.

– No = Initial number of individuals.

– λ = Geometric rate of increase.– t = Number of time intervals or generations.

Exponential Growth

• Continuous population growth in an unlimited environment can be modeled exponentially.

dN / dt = rmax N

• Appropriate for populations with overlapping generations.– As population size (N) increases, rate of

population increase (dN/dt) gets larger.

Exponential Growth• For an exponentially growing population,

size at any time can be calculated as:

Nt = Noert

• Nt = number individuals at time t.• N0 = initial number of individuals.• e = base of natural logarithms.• r (= rmax ) = per capita rate of increase.• t = number of time intervals.

2 types of pop growth

Exponential population growth

dN = rmaxNdt

Logistic population growth

dN = rmaxN (K-N)dt K

Population Growth Mathematically Defined

N=K/2

Logistic Population Growth

• As resources are depleted, population growth rate slows and eventually stops: logistic population growth.– Sigmoid (S-shaped) population growth curve.– Carrying capacity (K) is the number of individuals

of a population the environment can support.• Finite amount of resources can only support a finite

number of individuals.

Logistic Population Growth

dN/dt = rmaxN(1-N/K)

• rmax = Maximum per capita rate of increase under ideal conditions.

• When N nears K, the right side of the equation nears zero.– As population size increases, logistic growth rate

becomes a small fraction of growth rate.• Highest when N=K/2.• N/K = Environmental resistance.

Limits to Population Growth• Environment limits population growth by altering

birth and death rates.– Density-dependent factors

• Disease, Parasites, Resource Competition– Populations do not show continuous geometric increase

– When density increases other organisms reduces the fertility and longevity of the individuals in the population

– This reduces the rate of increase of the pop until eventually the pop ceases to grow

– The growth curve is defined as the sigmoid curve, S – shaped

– K = carrying capacity (upper asymptote or maximum value) – the maximum number of individuals that environment can support

– Density-independent factors• Natural disasters• Climate

Fig. 11.9

Galapagos Finch Population Growth

• Boag and Grant - Geospiza fortis was numerically dominant finch (1,200).

• After drought of 1977, population fell to (180).

• Food plants failed to produce seed crop.• 1983 - 10x normal rainfall caused population to

grow (1,100) due to abundance of seeds and caterpillars.

Cactus Finches and Cactus Reproduction

• Grant and Grant documented several ways finches utilized cacti:– Open flower buds in dry season to eat pollen– Consume nectar and pollen from mature

flowers– Eat seed coating (aril)– Eat seeds– Eat insects from rotting cactus pads

Cactus Finches and Cactus Reproduction

• Finches tend to destroy stigmas, thus flowers cannot be fertilized.– Wet season activity may reduce seeds

available to finches during the dry season.– Opuntia helleri main source for cactus

finches.• Negatively impacted by El Nino (1983).

– Stigma snapping delayed recovery.» Interplay of biotic and abiotic factors.

Human Growth

Fig. 11.24